The optical activity of cobalt(III) chelate diamine complexes

Abstract

The optical activity of the tris-diamine complexes of cobalt (III) in the visible region is accounted for quantitatively by a coulombic correlation between the components of the electric hexadecapole moment of the ¹ A ₁→¹ T ₁ d-electron transition of the cobalt(III) ion in the [CoN₆] chromophore and a transient electric dipole induced in each ligand group. The correlated electric dipole of the ligand group forms a non-zero scalar product with a component of the magnetic dipole moment of the cobalt(III) ion d-electron transition in the first order employing either an O or a D ₃ effective chromophoric symmetry. The calculated first-order rotational strengths account largely for the observed optical activity due to the chiral puckered conformation of the chelate rings and for the axial single-crystal circular dichroism of a cobalt(III) tris-diamine complex in a uniaxial crystal. The second-order rotational strengths improve the agreement and accommodate the observed circular dichroism of randomly-oriented cobalt(III) tris-diamine complexes. The increase in the dipole strength of the ¹ A ₁→¹ T ₁ transition in a tris-diamine complex, relative to [Co(NH₃)₆]³⁺, is explained by the model in the first order.